1. Field of the Invention
The present invention relates to a light emitting device including an element (hereinafter referred to as an EL element) in which a thin film (hereinafter referred to as an organic EL film) made of an organic compound capable of obtaining EL (Electro Luminescence) is interposed between electrodes.
2. Description of the Related Art
In recent years, a technique for forming a TFT (thin film transistor) on a substrate has been greatly developed, and its application and development to an active matrix type display device (light emitting device) has been advanced. Especially, since a TFT using a polysilicon film has higher field-effect mobility (also called mobility) than that of a conventional TFT using an amorphous silicon film, a high speed operation is achieved. Thus, it becomes possible to control a pixel, which has been conventionally performed by a driving circuit external to a substrate, by a driving circuit formed on the same substrate as the pixel.
In the active matrix type light emitting device like this, various circuits and elements are formed on the same substrate, so that various merits, such as reduction in manufacturing costs, miniaturization of an electro-optical device, improvement of yield, and reduction in throughput, can be obtained.
Moreover, research of an active matrix type light emitting device including an EL element as a self-luminous element has been made active.
The EL element has such a structure that an EL layer is interposed between a pair of electrodes (anode and cathode), and in general, the EL layer has a laminate structure. Typically, there is cited a laminate structure “hole transporting layer/light emitting layer/electron transporting layer” proposed by Tang et al. of Kodak Eastman Company. This structure has very high luminous efficiency, and most of what has been studied at present adopts this structure.
In addition, the structure of layers laminated in the order of a hole injecting layer/hole transporting layer/light emitting layer/electron transporting layer or a hole injecting layer/hole transporting layer/light emitting layer/electron transporting layer/electron injecting layer on an anode may be adopted. The light emitting layer may be doped with a fluorescent pigment or the like.
In the present specification, every layer provided between a cathode and an anode is generically called an EL layer. Thus, all of the foregoing hole injecting layer, hole transporting layer, light emitting layer, electron transporting layer, electron injecting layer and the like are included in the EL layer.
A predetermined voltage is applied to the EL layer of the above structure from a pair of electrodes, and then recombination of carriers is produced in the light emitting layer to emit light. Note that in the present specification, a light emitting element formed of an anode, an EL layer and a cathode is called an EL element.
Since the deterioration of the EL layer included in the EL element is accelerated by heat, light, moisture, oxygen and the like, in general, the EL element is formed after wiring lines and TFTs are formed in a pixel portion in the manufacture of an active matrix type light emitting device.
Various methods are proposed as forming (film formation) methods of the EL layer. For example, a vacuum evaporation method, a sputtering method, a spin coating method, a roll coating method, a cast method, an LB method, an ion plating method, a dipping method, an ink jet method, a printing method and the like can be cited.
Further, in order to realize colorization of the light emitting device, for example, as disclosed in Japanese Patent Application Laid-open No. Hei. 10-012377, there is proposed a technique in which an ink jet method is used to form EL layers with different luminous colors for respective pixels.
In order to realize a high definition display in the light emitting device, an active matrix type is desirable. However, in the case where a plurality of EL layers are separately painted and are formed to realize colorization, there is a case where poor formation of the EL layer occurs due to a metal mask shift by the minute structure.